It is generally known to provide cargo bags with lifting loops which are formed by extensions of the side walls so that they are easily accessible for engagement by the tines of a forklift truck or the hoist straps of a cargo handling crane. U.S. Pat. No. 4,191,229 discloses a cargo bag with two relatively wide lifting loops formed by the folded upper ends of adjacent side walls. These wide lifting loops restrict the size of the upper filling opening and are difficult to be engaged by the tines of a forklift truck or the like. U.S. Pat. No. 4,269,247 also discloses a cargo bag with a pair of lifting loops which are formed by the folded edge of the upper portions of the side walls. The lifting loops of this patent are lashed together by a rope or the like to form a single lifting loop extending upwardly therefrom. The manner in which the pair of lifting loops is formed by the folded upper ends of extensions of the side walls places a severe restriction on the size of the upper filling opening and also concentrates all of the lifting stress on the joined together lifting loops adjacent the center of the cargo bag.
U.S. Pat. No. 4,493,109 is directed to a cargo bag with a lifting loop positioned adjacent each of the four corners of the bag and the lifting loops are formed from sections of the side walls folded around reinforcing bands extending from the bottom to the top of the side walls. The cargo bag disclosed in this patent is difficult to fabricate from rectangular panels because it involves a complicated cutting and sewing operation and also includes seams at each corner, as well as seams in the medial portion of the opposite side walls which are subject to tearing apart when a heavy material is supported in the cargo bag. The cargo bag of U.S. Pat. No. 4,393,910 is formed of either a pair of rectangular panels or four individual rectangular panels folded transversely along a medial portion thereof to provide four integral lifting loops which are direct extensions of the side walls. The lifting loops are each one-fourth of the width of the container's upper filling opening so that they restrict the loading of material into the cargo bag. Also, the construction of the bag in accordance with this patent requires the use of either two or four vertical seams connecting together adjacent side wall panels and these connecting seams form weakened areas in the cargo bag.
U.S. Pat. No. 4,312,392 discloses a cargo bag formed of a single rectangular sheet of material with the upper edge portion being cut to provide a single lifting loop formed by joining together integral extensions of opposite corners of the cargo bag. The medial portion of the upper end of the cargo bag includes a filling opening which is restricted to approximately one-half of the upper circumference of the bag and, therefore, severely restricts the filling opening. Also, the single lifting loop provided in the bag of this patent concentrates the majority of the lifting stress in areas adjacent opposite corners of the bag.